In general, methods for forming multilayer coating films on automotive bodies are carried out by forming an electrodeposition film on an object to be coated, curing this film by heating and then forming a multilayer coating film comprising an intermediate coating film, a base coating film and a clear coating film. Recently, water-based coating materials have come to be used as intermediate coating materials and base coating materials in order to reduce the quantities of volatile organic components (VOCs) used and take into account the impact on the environment.
Furthermore, due to demands for reduced energy consumption in recent years, base coating films and clear coating films are formed on preheated intermediate coating films formed on electrodeposition film without curing the intermediate coating film by heating, and these three films are then simultaneously cured by heating. This is the so-called 3 coat 1 bake (3C1B) method and is a method that has come to be used as a method for forming multilayer coating films. For example, patent No. JP5734920 (patent literature 1), discloses a method of forming a multilayer coating film using a clear coating composition containing a core/shell-type emulsion composed of a core portion of acrylic resin and a shell portion of polyurethane resin and comprising a polyisocyanate, a hydroxy-group-containing acrylic resin, and water-based coating composition comprising a polyisocyanate and/or a polycarbodiimide as a curing agent. When this method of forming a multilayer coating film is used, it allows a decrease in the heating temperature for heat curing the coating film and further energy saving can be achieved. However, a primer may be required to ensure adhesion to plastic material when applying to plastic materials. Methods of coating the same coating on both steel-plate and plastic materials have been studied from the viewpoint of energy saving. For example, Japanese Laid-Open Patent Publication No. 2011-131135 (patent literature 2), discloses a multilayer coating film forming method using a monomer mixture containing a crosslinking monomer and a water-based coating material comprising an acrylic resin emulsion obtained by emulsion polymerization, a water soluble acrylic resin, a melamine resin and propylene glycol mono alkyl ether. However, it may not be possible to secure sufficient adhesion to plastic materials when applied to them if a primer is not applied in advance.
In addition, generally automobiles are forcefully hit by small stones and the like while in motion and there is the chance of local peeling of the coating. In coating film on steel plate, chipping resistance against peeling due to impact with such small stones is important and, in particular, the coating film is required to have excellent chipping resistance at low temperatures. For example, WO2011/010538 (patent literature 3) discloses a method of forming a multilayer coating film using a urethane resin emulsion having a specific acid value and weight average molecular weight produced with a polyisocyanate component and a polyol component as raw materials and a water-based primer composition comprising an oligomer having a specific number average molecular weight and a water tolerance of 10 or more. However, when the method of forming a multilayer coating film using this water-based primer composition is applied to plastic material, sufficient adhesion to the plastic material may not be achieved.
However, in relation to a method for forming a coating film, for example, Japanese Laid-Open Patent Publication No. 2009-39668 (patent literature 4) discloses a method for forming a bright coating film characterized by using a water-based primer coating composition containing a water-based non-chlorinated polyolefin resin, water-based polyurethane resin, water-based epoxy resin, internally cross-linked acrylic particle resin, and an emulsifying agent.
However, when this water-based primer coating composition is applied to electrodeposition-coated automobile steel plate, it may not be possible to achieve sufficient quality for rigorous chipping resistance at low temperatures.